a. Field of the Invention
The present invention relates to radio frequency (RF) communications systems, and in particular, to relatively large geographical area multifrequency communications networks for communication between mobile units and base stations.
b. Problems in the Art
Mobile RF data communication systems provide data communications in areas and at times when data communications might otherwise be impossible. Mobile RF data communication systems for surface transportation vehicles such as trains, for example, can provide equipment monitoring functions, freight monitoring functions and routing information exchange while the train is in motion. This is, of course, possible only with wireless communication, and this sort of communication provides continuous updating of such information, for example, as temperature, pressure and fuel levels of the train engine, maintenance information, and time for overhaul of the engine.
Freight monitoring functions can provide such information as temperature within refrigerated cars and location of freight contained within the train. This location information is important not only for inventory control, but also as a means for improving transportation efficiency.
RF data communication systems can also provide routing information. For example, the train can indicate which track it is on, as well as where on that track and in what direction it is going. In response, an office dispatch system connected to a network can reroute the train if there is a problem ahead using data communication via an RF base station. Alternatively, it may indicate to the train engineer to adjust speed for conditions ahead. It could also monitor and keep records of track use and train travel.
Other ground transportation systems, for example truck transportation systems, could also benefit from an RF data communication system. Again, the equipment monitoring, freight monitoring and routing information functions would improve efficiency, speed delivery, ensure better inventory control and reduce down time for the truck.
Mobile RF data communication systems for automobiles could also provide such things as rerouting information if for instance there is a detour on the road ahead or if traffic is especially heavy. Additionally, the system could be used to transmit updated information that works in conjunction with an onboard computer and satellite positioning systems to provide drivers with exact information about their location with respect to an onboard map. The map could be updated by the RF data communication system.
RF data communications networks could be advantageously employed in a similar advantageous manner for other situations. Any arrangement having mobile units through a geographic area of substantial size could benefit from the ability to communicate in a wireless manner.
A major impediment to implementation of a wide area, such as nationwide, RF data communication system is the fact that frequencies for such communication systems are allocated by the Federal Communication Commission (FCC) on a regional basis. It is therefore virtually impossible, for example, for a single network to acquire one frequency allocation which provides nationwide coverage. An exception to this general rule is the cellular phone system.
Cellular phone systems benefit from consistent nationwide frequency allocations by the FCC. Specific signal and voice channel assignments accommodate two phone systems in any area. Each cell within the local cellular service area is assigned one signal channel and multiple voice channels from a predetermined frequency allocation. Calls are initiated on the signal channel and the voice conversation occurs on a voice channel which will change from time to time as the vehicle moves from cell to cell during the phone call.
Cellular phone systems rely heavily on RF signal strength information in order to support the operation of the system, particularly the voice channel switching logic. This requirement, to precisely measure RF signal strength and to discriminate among several potential control actions based on the magnitude and difference in magnitude of this measurement, requires specialized equipment and control logic at both the base station and mobile device. Cellular systems also use this specialized equipment to measure signal strength in conjunction with the signal channel scan cycle. That is, while a mobile unit travels within a cellular phone service area, the mobile unit scans received RF signals and determines the proper RF frequency for the signal channel based upon the signal strength of the received RF signal. All such frequencies scanned are valid for cellular phone service nationwide.
The channel scan logic of the present invention does not require signal strength information, but rather, relies on the quality and reliability of data communications which, in the mobile environment at certain speeds and frequencies, has been shown to be only loosely correlated to signal strength leaving signal strength an unattractive criterion for frequency selection in the present invention.
Since one can not usually obtain licenses for a single frequency that covers the entire nation or other relatively large geographic area, networks must be composed of a patchwork of frequencies that, in composite, attempts to substantially blanket the entire nation or any geographical area of required coverage. Frequencies that may be valid for a mobile unit in one part of the network may be invalid for that mobile in another part of the network.
Even with a system such as this, there is a possibility that some areas will not be covered. It may be either by design, nature of terrain, equipment failure, down time for repair, or other reasons that some areas will not be covered. Further, there are some areas where coverage from more than one frequency might overlap and, due to the nature of RF communications at the boundaries of coverage, there may be areas of intermittent coverage.
It is therefore an object of the present invention to provide an improved system for combining regions of various frequency coverage into a national, or large geographic area, RF data communication network for transportation systems. The network acquisition mechanism is a key element for such a system and is the subject of the present invention. The network acquisition mechanism must provide a means for detecting when a mobile unit is in an area of coverage. It must discriminate between network and non-network communications, and may have to deal with areas in which one of several frequencies is the network frequency in that area. It must identify particular networks in areas where multiple networks share a frequency. It must also provide a means for determining when a mobile unit leaves an area of coverage and must also provide a relatively simple, inexpensive RF data system which can accommodate areas of overlapping coverage, areas of intermittent coverage, and a variety of frequencies assigned to the network on a regional basis.
Such a system must effectively function across a patchwork of frequency coverages. The FCC is very strict about maintaining the sanctity of those frequency assignments for certain areas. To do otherwise would make communication by these networks impossible and render their frequency licenses valueless. In order to comply with regional frequency assignments, truck drivers or cab drivers operating within large geographical areas, for example, may carry frequency coverage maps and compare those maps to their own location in order to determine what frequency to communicate on. Operators are strictly forbidden from transmitting on a frequency other than the one assigned to them in a particular area. A mobile operator cannot therefore use a hit and miss approach; transmitting on a frequency to see if it is the right one for the area, then transmitting on another, and so on, until he finds the proper frequency. Such a scheme would render larger areas of coverage useless due to the heavy traffic associated with operators searching for their assigned frequency. Another object of the present invention is, therefore, to eliminate the need for operator action in accessing an RF network and eliminating the need for on-board maps and geographical database.
These and other objects, features, and advantages of the present invention will become more apparent with reference to the accompanying specification and claims.